Might be useful to have a rectangular board that is 2x or 3x the width to allow you to connect shields side by side on top to the shield stack and processor below. Would be nice for use with a 7-segment number board or similar for display of numbers, etc.
Stack might be slightly thicker this way than side pins but one could use existing hardware.
Perhaps have an ability to rearrange some pins with jumpers or switches so that adjacent board conflicts might be minimized.
Glen
